No. 5/April 24, 1997
Areawide Corn Rootworm Project Update
In 1997, entomologists from the University of Illinois and Purdue University will cooperate in a joint program to manage densities of corn rootworms in a 16-square-mile area. Because you likely will hear more about this research effort over the next several years, the following questions and answers may help familiarize you with this large undertaking.
Where is the 16-square-mile area located? The study area is just south of Sheldon, Illinois, and is bordered by Highway 24 to the north, Indiana blacktop 71 to the east, and county roads 1300N and 2900E, to the south and west, respectively. The experiment crosses the state line and will be located partially in Iroquois County in Illinois and Newton and Benton counties in Indiana.
I've heard this is an effort to eradicate western corn rootworms. Is this true? No. The objective of the program is to cooperate with about 40 growers in the area in an effort to lower the density of corn rootworms to a level that is not economically threatening. The primary management strategy will be control of rootworm adults to prevent them from laying eggs, rather than application of soil insecticides at planting to prevent larval injury to roots. Cornfields will be scouted frequently, and when critical densities of beetles are reached, a treatment will be applied. By targeting insecticide applications at the egg-laying beetles, we predict that the overall density of corn rootworms in the 16-square-mile area can be lowered to a noneconomic level. This is not an attempt to eliminate beetles from the area.
Are similar efforts under way in other parts of the country? Yes. Three other corn rootworm areawide management projects also will begin this season. One study will be located in Kansas and involves close cooperation among scientists at Kansas State University and the University of Nebraska. Another experiment will take place in northwestern Iowa and involves cooperation among entomologists at Iowa State University, South Dakota State University, and the University of Minnesota. The third site is in Texas, but entomologists there will focus upon managing the Mexican corn rootworm, a species related to our western and northern corn rootworms.
Has areawide management of insects ever been successful? Yes. The most famous example of successful areawide management focused upon the screwworm, a potentially devastating insect pest of livestock. Dramatic success was achieved with the sterile male release strategy for screwworms in Florida and areas of the southwestern United States. Because the area in the Southwest is very large and nonisolated, constant reinfestations of screwworms from Central and South America have prevented total eradication of the screwworm. Some success also has been achieved with areawide management of boll weevils in the southeastern United States and areas of Arizona and California. Insecticide use for control of boll weevils declined significantly, while an expansion of cotton acreage occurred at the same time in these states.
Has an areawide management approach for corn rootworms ever been attempted? Yes. In the late 1960s, researchers in Nebraska attempted to suppress densities of adult corn rootworms within 16 square miles by applying ULV malathion in August of 1968, 1969, and 1970. Densities of adult rootworms were reduced by 39, 54, and 72%, respectively, for the three seasons. The Nebraska researchers concluded: "The program was successful to the extent that no economic infestations occurred in the treated area during any year following adult control, while use of soil insecticides was virtually abandoned in that area."
In 1991, 16 square miles near Brookings, South Dakota, were used to examine more closely the feasibility of suppression of corn rootworms. The South Dakota effort differed from the research conducted by Nebraska scientists in the late 1960s in the type of insecticide used. In the South Dakota experiment, a semiochemical-based bait (COMPEL, Scentry, Inc., Billings, Montana) was applied aerially to cornfields. The formulation consisted of biotac (45.5%, nontoxic adhesive), dried and ground roots of buffalo gourd (50%), and carbaryl (4.5%). The treatment was applied at a rate of 0.89 pound per acre to those fields that exceeded an economic threshold of one beetle per plant. The amount of active ingredient (carbaryl) applied was only 10 grams per acre, 98% less toxicant than the amount of toxicant in a typical application of a soil insecticide. Although densities of beetles were suppressed, evaluations of root injury the following season were not conducted. Thus, some critics of the areawide management approach for corn rootworms remain skeptical.
Why are corn rootworms being targeted in an areawide management program? If we accept the premise that pesticide reduction, specifically fewer "pounds in the ground," is one reason to examine areawide management more closely, then corn rootworms are an attractive candidate for this type of program. In the north-central United States, more than 60 million acres of corn are produced. Where corn is grown continuously (not rotated), a large percent of these acres is treated prophylactically (no scouting input) with a soil insecticide each spring at planting. Not surprisingly, the total amount of soil insecticide applied by producers each spring captures the attention of many agricultural and environmental policy makers. Proponents of an areawide management program for the corn rootworm complex see the potential for tremendous reductions in insecticide load across the Corn Belt if rootworm populations could be managed more effectively by integrating several tactics and not focusing solely on soil insecticides as a management tool.
Why was the Illinois and Indiana site chosen as one of the three pilot studies? The site was chosen primarily for the unique situation that exists with western corn rootworm beetles laying at least some of their eggs in soybean fields. Currently, this unique behavioral adaptation of the western corn rootworm is found only in certain areas of the eastern Corn Belt. The 16-square-mile study site is situated in the "heart" of the problem area. Due to the uniqueness of western corn rootworm egg-laying behavior in eastern Illinois and western Indiana, an insecticide will be applied to corn when a threshold is exceeded to prevent beetles from leaving the cornfield and laying eggs in nearby soybean fields.
What insecticide will be used in the Illinois/Indiana areawide project? The name of the product is SLAM. It is available commercially from BASF Corporation and consists of cucurbitacin, carbaryl, and nontoxic carriers. Cucurbitacin is a very bitter compound, derived from roots of buffalo gourd, that causes corn rootworm beetles to feed compulsively and arrests their movement. When cucurbitacin is used in conjunction with an insecticide like carbaryl, very little of the toxin is required to kill beetles. So, growers need only an ounce of carbaryl per acre instead of 1 pound of carbaryl, the typical application rate for products like Sevin XLR Plus.
What are some possible disadvantages with an areawide management approach for corn rootworms? One immediate concern is the potential for the development of resistance. Soil insecticides have been very good resistance-management tools in the battle with corn rootworms. Because soil insecticides are applied in a narrow band during planting, only a portion of the corn rootworm larval population is exposed to the toxin. In an areawide program, broadcast applications of SLAM will be directed against beetles. Over time, broadcast applications that target the entire population could select for insecticide resistance more quickly. For instance, in Nebraska, entomologists believe that growers in certain counties have selected for corn rootworm beetles that are resistant to Penncap-M (methyl parathion) through repeated (over many years) broadcast applications of this product aimed at the egg-laying adults.
There will be more questions as this areawide project in Illinois and Indiana moves forward. We will keep you informed as this effort unfolds and we begin to see results.
Mike Gray and Kevin Steffey, Extension Entomology, (217)333-6652